1. Field of the Invention
The present invention relates to a correction technique of an image forming apparatus for correcting a position of an image to be formed.
2. Description of the Related Art
Conventionally, there have been widely known copying machines, printers, and the like image forming apparatuses each of which uses an electrophotographic method to form color images.
In general, the temperature of an electrophotographic image forming apparatus rises due to heat from various motors including a polygon motor, a fixing heater, a power supply, and so forth, which act as heat sources after the startup of the apparatus, and/or change in ambient environment. In such an image forming apparatus, temperature rise in its apparatus body, particularly in a scanning optical device, causes variation in the irradiation position of a light beam, which is emitted from the scanning optical device, on the surface of a photosensitive drum, and as a consequence, the position of an electrostatic latent image formed on the photosensitive drum changes. For example, in an image forming apparatus which forms a color image by superimposition of images formed on a color component-by-color component basis, when the color-component images are positionally shifted from each other, color misregistration is caused in the color image. The variation of the irradiation position occurs depending on a change in the temperature of the image forming apparatus including the scanning optical device, and continues until the temperature of the image forming apparatus becomes constant.
Factors responsible for occurrence of the variation of the irradiation position include, for example, (a) change in the refractive index of a lens disposed in the scanning optical device and wavelength variation of a semiconductor laser due to temperature rise. The above-mentioned change and variation due to temperature rise change the irradiation position and characteristic values including total magnification. Also, locations of optical members including mirrors and lenses arranged in a casing (optical box) of the scanning optical device change due to (b) thermal expansion of the optical box, which can cause a change in the irradiation position of a light beam on the photosensitive drum. Also, the relative position between the photosensitive drum and a light beam can vary due to (c) thermal expansion of a support member for supporting the photosensitive drum. Further, the revolution speed of the photosensitive drum and the conveying speed of a transfer member can vary due to (d) expansion of a driving roller and the like, which sometimes causes variation in relative position between images formed by a plurality of image forming sections, respectively. Especially, the factors (a) and (b) are dominant ones crucial to the variation in the irradiation position. For this reason, there has been proposed an image forming apparatus that controls total magnification, a writing start position in the main scanning direction, a writing start position in the sub scanning direction, the inclination of a scanning line, and so forth, according to a change in the temperature of a scanning optical device. In this scanning optical device, e.g. a temperature in the vicinity of a polygon mirror is detected as the temperature of the scanning optical device.
As such an image forming apparatus, there has been proposed, for example, an image forming apparatus that detects the temperature of a scanning optical device by a temperature detecting element provided in the casing of the scanning optical device when the power is turned on, predicts the amount of shift of the irradiation position of a light beam based on the detected temperature, and controls the light beam based on the predicted shift amount (see Japanese Patent Laid-Open Publication No. 2006-11289).
However, the image forming apparatus proposed in Japanese Patent Laid-Open Publication No. 2006-11289 suffers from the following problem: In a state in which a polygon motor has not been warmed yet immediately after the start of rotation of the polygon mirror, the detected temperature is temporarily lowered by airflow generated by the rotation of the polygon mirror. For this reason, there is a possibility that the temperature detected immediately after the start of rotation of the polygon mirror becomes lower than the internal temperature of the scanning optical device. This makes it impossible to control, with high accuracy, the position of an image formed immediately after the start of rotation of the polygon mirror.